1. Technical Field
The present invention relates in general to wellhead assemblies and, in particular, to an improved system, method, and apparatus for forming a metal seal between inner and outer wellhead members.
2. Description of the Related Art
As shown in FIG. 1, seals 11 are used between inner and outer wellhead tubular members 13, 15 to contain internal well pressure. The inner wellhead member may be a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. The tubing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or tubing head. A packoff or seal seals between the tubing hanger and the outer wellhead member. Alternately, the inner wellhead member might be a casing hanger located in a wellhead housing and secured to a string of casing extending into the well. A seal or packoff seals in the annular space between the casing hanger and the wellhead housing.
A variety of seals of this nature have been employed in the prior art. Prior art seals include elastomeric seals 17 (see, e.g., FIG. 2) and partially metal anti-extrusion rings backing up elastomeric seal rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals are also employed. The seals may be set by a running tool, or they may be set in response to the weight of the string of casing or tubing. One type of prior art metal-to-metal seal 19 (see, e.g., FIG. 3) has a U-shaped cross-sectional shape with inner and outer walls separated by a conical slot. An energizing ring 21 is pushed into the slot to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members 13, 15. The energizing ring is a solid wedge-shaped member. The deformation of the inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
Thermal growth between the casing or tubing and the wellhead may occur, particularly with wellheads located at the surface, rather than subsea. The well fluid flowing upward through the tubing heats the string of tubing, and to a lesser degree the surrounding casing. The temperature increase may cause the tubing hanger and/or casing hanger to move axially a slight amount relative to the outer wellhead member. During the heat up transient, the tubing hanger and/or casing hanger can also move radially due to temperature differences between components and the different rates of thermal expansion from which the component materials are constructed. If the seal has been set as a result of a wedging action where an axial displacement of energizing rings induces a radial movement of the seal against its mating surfaces, then sealing forces may be reduced if there is movement in the axial direction due to pressure or thermal effects. A reduction in axial force on the energizing ring results in a reduction in the radial inward and outward forces on the inner and outer walls of the seal ring, which may cause the seal to leak. A loss of radial loading between the seal and its mating surfaces due to thermal transients may also cause the seal to leak.